Methods for reducing oxalic acid contents in plants

Abstract: Methods for reducing nitrate nitrogen and oxalic acid contents in plants, each of which comprises applying 5-aminolevulinic acid, a salt thereof or a mixture thereof. Nitrate nitrogen and oxalic acid contents in plants having high nitrate nitrogen and oxalic acid contents can be reduced effectively and easily.

Description: FIELD OF THE INVENTION This invention relates to methods capable of reducing, when applied to plants, the content of oxalic acid that spoils taste, nutrition and like qualities. BACKGROUND OF THE INVENTION Oxalic acid is an organic acid widely distributed in plants and takes part in the cause of the harsh taste of plants as a component of the "lye". In consequence, the oxalic acid content is one of the important factors which determine the quality of vegetables and the like. Especially, with the recently increasing occasion of eating vegetables raw in the form of salad and the like, great concern has been directed toward the reduction of the oxalic acid content in vegetables from the standpoint of taste. On the other hand, when vegetables are considered from the dietetics point of view, oxalic acid is not desirable because of its nature to inhibit absorption of calcium into the living body by forming an insoluble material through its bonding with calcium. Also, oxalic acid causes calculus because it occupies more than 60% by weight of the urinary stone components. In addition, since incorporation of oxalic acid into human body originates mostly from vegetables, it is important to reduce the oxalic acid content in vegetables. In the conventional cooking of spinach, komatsuna (a kind of Chinese cabbage) and the like, their oxalic acid content is reduced by the so-called "removal of harsh taste" in which water is allowed to slop over in boiling them, but merely about 20% of the total amount of oxalic acid is reduced by such a means. Though attempts have been made to produce vegetables with low oxalic acid content by selecting appropriate varieties and controlling fertilizer application, harvesting period and the like, satisfactory results have not been obtained yet. Also, very little actually is known about success in the development of a chemical agent which can reduce the oxalic acid content in vegetables. SUMMARY OF THE INVENTION In view of the above, it is therefore an object of the present invention to provide a method which can reduce the content of oxalic acid in plants. Taking the aforementioned problem involved in the prior art into consideration, the inventors of the present invention have conducted intensive studies with the aim of finding a chemical agent which can reduce the content of oxalic acid in plants and found unexpectedly that 5-aminolevulinic acid, its salt or a mixture thereof functions to reduce the content of oxalic acid in plants to a satisfactory level. The present invention has been accomplished on the basis of this finding. Accordingly, the present invention provides methods for reducing oxalic acid content in plants each of which comprises applying 5-aminolevulinic acid, its salt or a mixture thereof. Also, the present invention provides chemical agents for reducing oxalic acid content in plants, each of which comprises 5-aminolevulinic acid, its salt or a mixture thereof. In this connection, the present inventors already have proposed a method for applying 5-aminolevulinic acid or a salt thereof to plants by which growth, rooting, photosynthetic activity and the like of plants is promoted (U.S. Patent 5,298,482). In addition, an oxalic acid reducer containing vitamin B 6 is described in JP-A-4 297 402-A However, the effect of reducing the content of oxalic acid in plants as disclosed herein is a newly found effect which cannot be expected from the previously proposed growth-promoting and the like effects. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 5-Aminolevulinic acid or a salt thereof to be used as the active ingredient of the present invention is a known natural compound which can be produced by any known method such as chemical synthesis, microbial production, enzymatic synthesis, extraction from natural sources or the like. In the case of the microbial production for example, the fermentation product may be used as it is without employing separation and purification steps, provided that the product does not contain substances which spoil the effect of the present invention. Examples of the salt of 5-aminolevulinic acid include acid addition salts such as the hydrochloride, phosphate, nitrate, sulfate, acetate, propionate, butyrate, valerate, citrate, fumarate, maleate, malate and the like salts and metal salts such as the sodium, potassium, calcium and the like salts. In order to effect a reduction in the content of oxalic acid in plants by the method of the present invention, any treating method such as foliar application, soil treatment or the like may be used, provided that 5-aminolevulinic acid or a salt thereof is absorbed by plants. Any treating or application method can be used as long as the method produces the plant with the ability to absorb the 5-aminolevulinic acid or salt thereof. In the case of hydroponics and the like, 5-aminolevulinic acid may be used in the form of an aqueous solution to achieve root absorption. When the method of the present invention is used in foliar application, it can contain preferably from 1 to 1,000 ppm, more preferably from 5 to 500 ppm, of 5-aminolevulinic acid, and it can be applied in an amount of preferably from 10 to 3,000 l, more preferably from 20 to 1,000 l, per 1,000 m 2 . When the method of the present invention is applied to a plant whose leaves can hardly retain the active ingredient, it is preferable to use a spreader together with 5-aminolevulinic acid. The type and amount of the spreader are not particularly limited and can be determined appropriately in a conventional manner. When the method of the present invention is used in soil treatment, it can be applied to soil in an amount of preferably from 0.5 to 800 g, more preferably from 1 to 300 g, of 5-aminolevulinic acid per 1,000 m 2 . When the method of the present invention is to be absorbed through roots of plants in the form of an aqueous solution such as the case of hydroponics, it contains preferably from 0.001 to 50 ppm, more preferably from 0.01 to 20 ppm, of 5-aminolevulinic acid. 5-Aminolevulinic acid exerts its effect satisfactorily by a single operation of any application process described above, but the effect is improved further by repeating the process several times. Though not particularly limited, the application may be carried out preferably at an early stage of plant growth. 5-Aminolevulinic acid may be used in the form of a mixture with other agricultural chemicals, fertilizers and the like, provided that they do not adversely affect of the present invention. With regard to a method for reducing the oxalic acid content in plants, as another one of the methods of the present invention, plants to be treated with this method are not particularly limited, but its effect becomes markedly evident when a plant having a high oxalic acid content, preferably 50 mg or more of oxalic acid per 100 g of the plant, more preferably 100 to 5,000 mg of oxalic acid per 100 g of the plant, is treated. Illustrative examples of such plants include vegetables such as spinach, komatsuna (a kind of Chinese cabbage), garland chrysanthemum, sunny lettuce, celery, salad and the like, pasture grasses such as maize, clover and the like and weeds such as a broad-leaved plantain, a Japanese knotweed, a wood sorrel, a sorrel and the like. The following examples are provided to further illustrate the present invention. It is to be understood, however, that the examples are given for purpose of illustration only and are not construed as limiting the scope of the present invention. Unless otherwise indicated herein, all parts, percents, ratios and the like are by weight. EXAMPLE 1 On October 20, 11 spinach seeds (Glory, available from Takii Shubyo K.K.) were sowed in a pot of 1/500,000 m 2 filled with field soil. As a basal fertilizer, 1.5 g of a compound fertilizer Sosai No. 3® (N:P 2 O 5 :K 2 O = 15:15:15) was applied to each pot but with no additional dressing. As the seedlings grew under usual tending conditions in a greenhouse, thinning was repeated to select 3 uniform seedlings to be grown further in each pot. On November 11, foliage treatment was carried out with 20 ml per pot of an aqueous solution containing 0, 50, 100 or 300 ppm of 5-aminolevulinic acid hydrochloride (hereinafter referred to as "5-ALA") and 0.1 ml/100 ml of a spreader (Approach® (mainly composed of 50% of polyoxyethylenehexytane fatty ester and 20-30% of isopropyl alcohol), manufactured by Kao Corp.). Each treatment was carried out using 6 pots. After further tending under usual conditions in the greenhouse, the plants were harvested on January 18 to measure the oxalic acid content in the leaves using a high performance liquid chromatography (HPLC) in a conventional manner. The results obtained are shown in Table 1 below. 5-ALA Conc. (ppm) Total Oxalic Acid (mg/100 g FW ) 0 665.0 50 335.0 100 420.0 300 335.0 As is evident from the results shown in Table 1 above, the oxalic acid content is greatly reduced by the treatment of the foliage with the agent of the present invention. The thus harvested spinach plants were washed with water and subjected to sampling without cooking to examine "harsh taste". The results obtained are shown in Table 2 below. In this case, the panelists were not given any information about the spinach oxalic acid content, 5-ALA concentration and the like. 5-ALA Conc. (ppm) Panelists A B C 0 o ○ o o 50 o ▵ X 100 o ▵ X 300 o ▵ X o ○: very strong in "harsh taste" o: strong in "harsh taste" ▵: weak in "harsh taste" X: negligible in "harsh taste" As is evident from the results shown in Table 2 above, "harsh taste" at the time of eating the uncooked vegetable is reduced by the method of the present invention, thus confirming the effect of the agent to reduce the so-called "lye". EXAMPLE 2 Spinach plants (Glory, a western variety; available from Takii Shubyo K.K.) which had been cultivated and treated in the same manner as described in Example 1 were subjected to sampling to evaluate their "harsh taste". Panelists participated in this test were first- to third-year students at an elementary school who dislike spinach. The results are shown in Table 3 below. In this case, the panelists were not given any information about the spinach oxalic acid content, 5-ALA concentration and the like. 5-ALA Conc. (ppm) Panelists A B C D 0 o o o o 50 o ▵ ▵ o 100 o ▵ X ▵ 300 ▵ ▵ X X o: too bitter to eat ▵: bearable to eat X: delicious As is evident from the results shown in Table 3 above, the taste of the western spinach inherently having high oxalic acid content is improved by the method of the present invention to such a level that even children who dislike spinach can eat it. EXAMPLE 3 On October 20, 11 komatsuna seeds (Osome, available from Takii Shubyo K.K.) were sowed in a pot of 1/500,000 m 2 filled with field soil. As a basal fertilizer, 2.5 g of a compound fertilizer Yasai No. 3® (N:P 2 O 5 :K 2 O = 15:15:15) was applied to each pot but with no additional dressing. As the seedlings grew under usual tending conditions in a greenhouse, thinning was repeated to select 3 uniform seedlings to be grown further in each pot. On November 11, foliage treatment was carried out with 10 ml per pot of an aqueous solution containing 0, 50, 100 or 300 ppm of 5-ALA and 0.1 ml/100 ml of a spreader (Approach, manufactured by Kao Corp.). Each treatment was carried out using 6 pots. After further tending under usual conditions in the greenhouse, the plants were harvested on January 18 to measure the oxalic acid content in leaves using HPLC in a conventional manner. The results obtained are shown in Table 4 below. 5-ALA Conc. (ppm) Total Oxalic Acid (mg/100 g FW) 0 185.0 50 85.0 100 92.0 300 78.0 As is evident from the results shown in Table 4 above, the oxalic acid content in komatsuna is reduced to half or more by its treatment with the agent of the present invention.

Claims: A method for reducing oxalic acid content in a plant, which comprises applying 5-aminolevulinic acid, a salt thereof or a mixture thereof. The method for reducing oxalic acid content in a plant according to claim 1, wherein a plant to be treated has an oxalic acid content of 50 mg/100 g plant or more. The method according to claim 1 or 2 wherein said salt of 5-aminolevulinic acid is at least one salt selected from the group consisting of the hydrochloride, phosphate, nitrate, sulfate, acetate, propionate, butyrate, valerate, citrate, fumarate, maleate, malate, sodium, potassium and calcium salts. The method according to any one of claims 1 to 3, wherein a spreader is further present. The method according to any one of claims 1 to 4, which comprises treating foliage of said plant with 1 to 1,000 ppm of 5-aminolevulinic acid, a salt thereof or a mixture thereof. The method according to claim 5, wherein said plant is treated with 10 to 3,000 l per 1,000 m 2 of said compound. The method according to any one of claims 1 to 3, which comprises applying 5-aminolevulinic acid, a salt thereof or a mixture thereof to soil in which the plant is grown in an amount of 0.5 to 800 g/1,000 m 2 . The method according to any one of claims 1 to 3, which comprises hydroponically culturing said plant using a culture solution containing 0.001 to 50 ppm of 5-aminolevulinic acid, a salt thereof or a mixture thereof.

Inventors: HOTTA YASUSHI, TANAKA TOHRU, YOSHIDA RYUJI, HOTTA, YASUSHI, TANAKA, TOHRU, YOSHIDA, RYUJI, HOTTA, YASUSHI, C/O COSMO RESEARCH INSTITUTE, TANAKA, TOHRU, C/O COSMO RESEARCH INSTITUTE
Applicants: COSMO OIL CO LTD, COSMO SOGO KENKYUSHO KK, COSMO OIL CO., LTD, COSMO RESEARCH INSTITUTE
Application year: 1994
Main classification: A01N37
IPC codes: A01N37